All of the material in this patent application is subject to copyright protection under the copyright laws of the United States and of other countries. As of the first effective filing date of the present application, this material is protected as unpublished material. However, permission to copy this material is hereby granted to the extent that the copyright owner has no objection to the facsimile reproduction by anyone of the patent documentation or patent disclosure, as it appears in the United States Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever.
Not Applicable
1. Field of the Invention
This invention generally pertains to testing of signal transmission lines, and more particularly to the field of testing dual conductor transmission lines used in differential signaling.
2. Description of the Related Art
Progress in the field of computing and related to the Internet during the last decade has spurred increased interest in digital signaling technology and placed increased demands on the global telecommunications infrastructure. Some of the newer parts of the telecommunications infrastructure used for the Internet, such as transatlantic fiber optic cables are inherently digital. Older parts of the telecommunications infrastructure that must be used at present, such as the so called xe2x80x98last milexe2x80x99 connection of individual homes to the public telephone system through twisted pairs, and cable TV distribution networks were designed as analog systems, and are now being adopted for digital use-for the Internet. Digital signaling through transmission lines originally intended for analog use, such as twisted pairs or coaxial cable places higher demands on the integrity of those systems. Imperfections that are tolerable for analog use are not acceptable for high baud rate digital use.
Differential signaling is a technique that can be used to transmit digital signals through dual conductor transmission lines such as twisted pairs of wires formerly used for audio bandwidth analog signals. Differential signaling, as opposed to using a single transmission wire referenced to ground separately at both ends, has the advantages that it is less prone to common mode noise, and variations in the ground potential (ground path noise) between the transmitting and receiving station.
One known scheme for differential signaling is where a transmitter applies a first voltage to a first line and a second voltage to a second line to transmit a logical zero signal. To transmit a logical one signal, the first voltage is placed on the second line and the second voltage is placed on the first line. At the receiver the voltages on the two lines are compared. The decode scheme could be such that if the voltage on the first line exceeds that on the second by a predetermined amount a binary one is construed, and in the opposite case, a binary zero is construed. The predetermined amount must be some finite amount such that noise level variations between the voltage on the two lines are not construed as signals.
A second known technique for differential signaling is one in which one wire is used to connected a ground voltage reference between the receiver and the transmitter, and a second wire carries a voltage which is set within one of two specified ranges to transmit either a binary zero or binary one.
The length of the physical line varies from one system to another, and there may be a number of interconnected physical lines which form the signal pathway in some systems. The varied length along with variation in the quality of the interconnection, i.e. ohmic losses, or impedance mismatches, lead to a variation in the signal voltage levels, and consequently the signal voltage difference, reaching the receiver. There is also some finite noise amplitude which is added to the signal. Accordingly receivers for decoding differential signal encoded binary data are designed to accept a certain range of voltage differences as representing a binary 1 and a range as representing binary 0. For a short link made of one unbroken run of wire, the voltage appearing at the receiver may be high, and conversely, for a long link having many interconnected wires the voltages may be low. The fact that there is a range of voltages that are interpreted as a binary 1 and a range interpreted as a binary 0 can result in a faulty line functioning normally under low noise condition but causing data errors in actual use. If due to a faulty connector or other reason, there is an open in one line, and if the system is not subject to common mode noise or ground path noise, then the system may still function normally, although the signal reaching the receiver would be near the lower end of the specified ranges. However in the case where a system with a fault is subject to common mode noise, propagating through the system from the receiver or the common mode noise on the signal line is present by induction from nearby signal lines or equipment, then the common mode signal will be stopped at the open in the wire and only propagate on the other signal wire, thus appearing at the receiver as a differential signal. Depending on its magnitude and polarity it may fall within one of the ranges corresponding to a binary one or binary zero, and corrupt the data stream. This kind of intermittent fault is difficult to detect and thus very problematic for manufactures of equipment that uses a large number of these type of data links. During the pre-shipment test of equipment, the equipment may not be subject to electromagnetic interference that would cause common mode noise and any open circuits such as discussed above would not cause erroneous signals until the equipment has been set up at an electrically noisy customer site.
At present transmission lines can be diagnosed by applying a signal generator to the one end of the transmission line and examining the signal received at the other end of the line with a signal. This requires a great deal of time spent by a skilled technician to set up each line for testing.
Accordingly, there is a need for a means to assuredly test for opens and shorts in the conductors that comprise the differential mode data links. It would be desirable to have a test circuit that is simple enough to be added to the signal receiver without excessively increasing the cost. Such a test circuit must be designed in such a way that it does not degrade the performance i.e. bandwidth of the data link.
According to one aspect of the invention an auxiliary line driver applies first test signals to a receiver end of a transmission line, a data transmitting line driver applies second test signals at a transmitting end of the transmission line, and a receiver amplifier-comparator at the receive end is used to read a voltage resulting at the receiver inputs from the interaction of the first and second test signals with the impedances of the transmission line impedances, the line driver output impedances and the impedance of a terminating resistor, the voltage being interpretable according to the teachings set forth hereinafter to indicate the presence of shorts or opens in the transmission line.
According to a further aspect of the invention a two conductor signal line is provided with a line driver at a first end, a receiver amplifier at a second end, and a test circuit at the second end, the test circuit comprising, a test signal input, a first buffer adopted to receive a test signal at a first input pin, and to drive a first conductor of the two conductor signal line from the second end, based on the input test signal, an inverter adopted to receive the test signal, and output an inverted test signal to a second buffer, the second buffer driving a second conductor of the two conductor line from the second end, whereby a binary one or zero test signal can be applied to the test signal input to cause the test circuit to drive one of the two conductors high and the other low depending on the logical value of the test signal.
According to a further aspect of the invention the buffers are of the three state logic type the outputs of which can be put set to an open state during normal use of the data link so as not to interfere with the normal data flow.
According to a further aspect of the invention the auxiliary signal driver, when the outputs are not set open, are characterized by an output impedance that is high relative to ohmic conduction loss impedance (DC impedance) of the transmission line.
According to another aspect of the invention the above described circuit is used in the following manner. A first test signal is applied to the line driver to cause it to apply a first set of prescribed signals to the first and second conductors at the first end, and second test signal is applied to the test circuit so as to cause the first and second buffers to apply a second set of voltage signals to the first and second conductors at the second end, and an output of the receiver is produced from the interaction of the first and second test signals applied to the first and second ends, and an impedance network made up of the first and/or second conductors, one or more line terminating resistors, output impedances of the first and second buffers, and output impedances of the first line driver. The output of the receiver can be interpreted based on the teachings of this invention to indicate opens in the first and second conductors, and/or shorts between one or more system supply voltages and at least one of the first and second conductors through the line driver.